The process of cellular differentiation often involves a tightly regulated network of structural and functional changes within a cell. Changes occurring during differentiation can be regulated on the level of protein activity as well as on the level on gene expression. In a variety of cell types including epidermal and neuronal cells, the activity of cytoskeletal proteins changes drastically during differentiation. Another example of significant cytoskeletal re- arrangements occurs in the spermatozoa of the nematode Caenorhabditis elegans. During the differentiation process known as spermiogenesis, nematode spermatozoa undergo a significant morphological change from spherical cells to crawling, asymmetric cells. The movement of these cells is fueled by the dynamic action of MSP filaments within a membranous extension referred to as the pseudopod. MSP (major sperm protein) comprises 15% of the total protein within C. elegans sperm and is the main cytoskeletal component in the spermatozoa. Regulation of MSP assembly is a crucial step in the differentiation of C. elegans sperm; however the factors/gene products regulating MSP assembly have yet to be identified. The current focus of my project is to identify some of the cellular proteins directly involved in MSP assembly. Presently, I am using the Yeast Two-Hybrid system to screen for genomic proteins that interact with MSP. Proteins identified in this screen will be tested in a variety of assays for their affiliation with MSP both in vitro and in vivo. Information gained from the results of these studies will provide insights into this interesting mechanism of MSP based cell motility.